In the fields of remote data collection and processing, electronic article surveillance and automatic identification are most successfully accomplished by employing an interrogator that generates an electromagnetic signal throughout a limited space. The electromagnetic signal is received, modified to include identification information and reradiated by a transponder within a tag carried by each article in the radiated space. If conditions are acceptable, the interrogator receives the reradiated signal; if not, no identification or an incorrect identification result.
Prominent in effecting reliable, correct identification is the performance of the antennas utilized by the interrogator and the tag. Unfortunately, in many applications the articles carrying the tags often position the tag antenna while in the interrogation space in an orientation creating a polarization mismatch between the antennas, effectively substantially reducing energy transfer therebetween.
Efforts have been made to reduce the extent of polarization mismatch or otherwise increase the coupling between the antennas of the interrogator and tag. Many of these efforts have centered around configuring the interrogator antenna to monitor for multiple polarizations by either phasing the interrogator antenna or using multiple antennas of differing polarizations. It has also been suggested that the reradiated signal from the tag antenna be circularly polarized. However, phased or multiple antenna configurations are as large or larger than planar polarized antennas, more costly and difficult to manufacture and operate, and have poor gain and directivity. Moreover, the physical constraints that applications place on tag characteristics make manufacture of a circularly polarized tag antenna impracticable and equally performance limited.
Both the interrogator antenna and tag antenna should be as small as possible and simple and easy to manufacture. Equally important, the interrogator antenna should provide a hemispherical radiation field with good gain in the forward direction and low gain to the sides and back in order to facilitate optimal coupling to the tag antenna whose polarization orientation may change at random as the article carrying the tag moves through the interrogation space. The tag antenna should directly couple to the transponder circuitry with which it operates with a high degree of impedance matching.